This invention relates to apparatus that can be used with a stereo decoder and, more particularly, to a balanced synchronous detector circuit which is adapted to receive a pilot signal and to detect the amplitude thereof and also to control a local oscillator to generate a local oscillating signal that is phase-locked with the pilot signal.
A typical type of stereo decoder is described in U.S. Pat. No. 3,842,211. In that decoder, a composite stereo signal composed of a main-channel (L+R) signal, a sub-channel (L-R) signal modulated onto a sub-carrier of 38 kHz and a pilot signal of 19 kHz is received and decoded into separate left and right channel signals L and R. The decoding of the composite stereo signal is dependent upon demodulating the sub-channel signal, and this generally requires the generation of a local sub-carrier of 38 kHz. Typically, the local sub-carrier is produced by a phase-locked circuit wherein the phase and frequency of a local oscillator are controlled by phase detection of the pilot signal included in the composite stereo signal. That is, the frequency of the local oscillator is divided to a frequency equal to that of the pilot signal and the phase of the divided local oscillator signal is compared to that of the received pilot signal. Any phase differential therebetween is used to adjust the local oscillator. Also, in this type of stereo decoder, the presence or absence of a stereo broadcast is determined by detecting the amplitude of the received pilot signal. That is, if the pilot signal amplitude exceeds a predetermined threshold, it is assumed that a stereo signal has been received.
In prior art stereo decoders, the pilot signal amplitude detector and the phase detector usually are constructed of individual circuits, each such circuit being provided with an amplifier to amplify the pilot signal accordingly. Also, although a 38 kHz sub-carrier is needed to decode the received sub-channel signal and a 19 kHz local signal is needed for phase comparison with the received pilot signal, the phase detector heretofore has required a local oscillator, or reference, signal whose phase differs from the phase of the received pilot signal by 90.degree. . However, the pilot signal amplitude detector normally must be supplied with a locally generated reference signal that is in phase with the received pilot signal. Hence, if both of the locally generated reference signals are derived from a common local oscillator whose oscillating frequency is greater than either reference signal, the frequency dividers which must be used to produce the locally generated reference signals are required to impart a relative phase shift of 90.degree. therebetween. Hence, prior art proposals for phase detecting circuitry and for amplitude detecting circuitry generally have resulted in complex circuit construction. Also, amplitude detectors which have been used heretofore are susceptible to erroneous operation in the event that a noise component is received having a frequency that may be similar to the pilot signal frequency. That is, such noise signal may be erroneously interpreted as a pilot signal.